A pediatric perspective on the unique vulnerability and resilience of the embryo and the child to environmental toxicants: the importance of rigorous research concerning age and agent.

There is realistic concern about the impact of environmental influences on the health of human populations. First, exposure to environmental agents continues despite successes in reducing exposures to known toxicants such as lead, polychlorinated biphenyls (PCBs) and tobacco smoke. Second, there has been increasing concern about the cause of autism and other neurodevelopmental problems and hypotheses that environmental influences may play a role in the prevalence of these and other such childhood and adult conditions as asthma and obesity. Third, many other conditions are directly or indirectly related to environmental influences and are preventable, such as injuries, untoward consequences of alcohol, suicide, drug addiction, and gun-related deaths. There have been numerous publications since the 1970s of symposia, proceedings, monographs, and articles dealing with the increased susceptibility of the embryo, infant, and child to environmental toxicants,1–17 reflecting a greater level of concern about embryonic and childhood exposures. Indeed, great deal of attention has been paid to the vulnerability of the embryo and the fetus to environmental chemicals, drugs, and physical agents. In fact, the publication edited by Miller1 was primarily devoted to exposures to the embryo and the fetus. Because the embryo and the child are growing and their tissues and organs are differentiating, deleterious effects may occur at lower exposures to some chemicals, drugs, and physical agents and produce more severe effects than those seen in adults. In fact, some effects may not occur in adults. Thus, maximal permissible exposures (MPEs) for some environmental chemicals should be lower for the embryo and the child. It is important to note that children and adolescents have better recuperative capacities than adults for many toxic agents, and, similarly, appropriate drug dosages may be lower or higher on a mg/kg or surface area basis in children than in adults to attain effective therapeutic blood levels or to avoid toxicity. In addition, effects produced by drugs, chemicals, and physical agents are not always deleterious or always irreversible. This means that for some exposures, the young can recover from some effects more rapidly and completely than adults (Table 1). If the exposure does result in irreversible effects by exceeding the threshold exposure, then the impact on a developing organism can be more severe than in the adult. Much of the discussion and publications that deal with the vulnerability of the developing embryo, infant, child, and adolescent to environmental agents have focused on particular environmental toxicants or agents, summarizing the spectrum of pathology that results from exposures to these agents. There is nothing wrong with this approach from the toxicologist’s point of view, because it is obvious that the developing child and adolescent can be more severely or differently affected by some environmental toxicants.